A hydrokinetic water turbine system includes a frame structure, first and second shafts rotatably supported by the frame structure, and first and second rotors secured to the first and second shafts respectively and each having a plurality of spaced-apart blades so that the flowing stream of water revolves the first and second rotors about a central axis of the first and second shafts respectively. The first and second shafts are horizontally-disposed and coaxial and the first and second rotors are adapted to rotate in opposite directions. The blades are hollow and filled with a foam material to reduce weight and increase buoyancy. The frame structure is a substantially rectangular and open frame structure and includes frame members adapted to reduce a coefficient of drag of the frame structure. The frame members can be hollow and filled with a foam material to reduce weight and increase buoyancy.